1. Field of the Invention
The present invention relates generally to cellular communication systems, and more specifically to dynamic assignment of channels to mobile stations from a base station using an array of antennas respectively covering sectors of the service area of the base station.
2. Description of the Related Art
The capacity of a cellular mobile communication system may be increased by reducing the size of the cells so that the total number of channels available per unit area is increased. This is achieved in practice by the process of "cell splitting" where a new base stations are established. As the cells are split into smaller sizes, the co-channel interference from distant cells increases. One way of reducing the level of co-channel interference is to use directional antennas at base stations, with each antenna illuminating or covering a sector of the cell, whereas ominidirectional antennas used in non-sectorized cells.
Dynamic channel assignment schemes are known for efficient utilization of channels, and used for systems of non-sectorized cells. For sectorized cells, a dynamic channel assignment scheme is described in a paper "Channel Segregation, A Distributed Adaptive Channel Allocation Scheme for Mobile Communication Systems", Y. Furuya et al., Second Nordic Seminar on Digital Land Mobile Radio Communication, Stockholm, 1986, pages 311-315. According to the proposed scheme priorities are assigned to a list of available channels according to past records of co-channel interference. On receiving a request from a mobile station, a channel is selected from the list according to the priorities so that the mobile station has a reduced chance of encountering co-channel interference. A dynamic channel assignment scheme for efficient utilization of channels in sectorized cells is described in a paper "Adaptive Channel Allocation in a TIA IS-54 System", H. Andersson et al., Vehicular Technology Conference, 1992, pages 778 to 781. According to this scheme, carrier-to-interference ratio is used criteria for assigning priorities to available channels.
However, there is Still a need for improvements in dynamic channel assignment scheme for sectorized cells in terms of the utilization of channels, the number of channel selection attempts for each request, and the probability of co-channel interference. More specifically, assume that, in a sectorized cellular communication network, first and second mobile stations are respectively communicating with first and second base stations. The signal from the first base station may be received by the distant, second mobile station as an undesired downlink signal and produce a significant level of interference, while the signal from the first mobile station may be received by the second base station as an undesired uplink signal. If the directional antenna of the second base station which is being used for the second mobile station is oriented in a direction away from the first base station, the undesired uplink signal from the first mobile station will produce a low level of interference. Since interference is currently detected only from a signal received by the antenna used for a mobile station of interest, the cross correlation for co-channel interference between the uplink and downlink signals of the mobile station is low.
Because of the low cross correlation, the current dynamic channel assignment schemes are not satisfactory in respect of the utilization of channels, the number of channel selection attempts for each request, and the probability of co-channel interference.